Keratoconjunctivitis sicca (KCS), or "dry eye" is an eye disorder caused by an absolute or partial deficiency in lacrimal gland fluid production. This disorder is one of the most common complaints in ophthalmology. Normally, the exposed portion of the eye is covered by a thin layer of fluid, or tear film, supplied by the lacrimal gland. This fluid is important for the well-being of the corneal and conjunctival epithelium and provides the cornea with an optically high quality surface. Lacrimal fluid acts as a lubricant between the ocular surface and the eyelids and contains proteins such as immunoglobulin A, lysozyme and beta lysin, which are known to have bacteriostatic properties. One component of the tear film, mucin, helps the tears to flow evenly over the eye and is produced by specific cells that are stimulated by specialized nerves. Insufficient production of lacrimal gland fluid is one of the most frequent causes of "dry eye".